A variety of optical detection systems have been developed which are sensitive to radiation of various wavelength ranges. For example, digital cameras are commercially available to consumers and are configured to detect and record reflections and emissions of light having a wavelength in the visible spectrum range, thereby effectively capturing and recording scenes (as a human observer might sense them). As another example, infrared imaging systems are provided to detect radiation in the infrared wavelength range that is emitted from or reflected by objects within a scene. Such infrared imaging systems can view objects within scenes that would normally not be apparent to an optical detection system capable only of detecting light in the visible spectrum. These infrared imaging systems are particularly useful for real-time viewing of scenes at night or through smoke, and are frequently utilized in military equipment for detecting missiles, aircraft, vehicles, vessels, and the like.
Such systems typically utilize a focal plane array for detection of radiation. A focal plane array comprises an array of pixel locations, wherein each pixel location includes one or more minute detectors configured to detect a portion of the radiation emitted from the scene. The array is coupled to an integrated circuit, and the resulting image is formed by the combination of the portions detected at the various pixel locations. In other words, the simultaneous signals from the various detectors at the pixel locations of the array provide a representation of the scene in real-time.
When a single pixel of a focal plane array includes a plurality of detectors, each of the included detectors can be configured to detect a different color of light. For example, in a focal plane array configured to detect visible light, each pixel might include four detectors, wherein one detector is configured to detect red light, one detector is configured to detect blue light, and two detectors are configured to detect green light. Similarly, a focal plane array configured to detect infrared light might include two detectors, wherein a first detector can detect infrared light within a first wavelength range (e.g., between 3.5 μm and 4.0 μm) and the second detector can detect infrared light within a second wavelength range (e.g., between 4.3 μm and 4.8 μm. A focal plane array having either such configuration can therefore differentiate the incoming light and can consequently identify the wavelength(s) of the incoming light. In both visible and infrared imaging systems, such discrimination of wavelength ranges or “colors” is desirable because it provides additional information about the source of the radiation and can assist in identifying the objects being viewed.
To implement such a color-sensitive focal plane array, a “stacked,” three-dimensional detector configuration could be implemented wi specifically, multiple detectors can be placed vertically on top of one another such that the top detector can detect the portion of light within a first wavelength range but can allow light outside of the first wavelength range to pass through to lower sensors configured to detect such light. In this manner, multi-band radiation can be detected and discriminated. However, some such “stacked” architectures can require etching of complex vias in the device, and can also require complex procedures for aligning the detectors and readout circuitry. Such high complexity can result in high production costs and lowered operability and reliability.
In other implementations, flat architectures could be utilized where detectors are not stacked but rather are placed side-by-side horizontally within a two-dimensional array. However, such multi-color, two-dimensional architectures typically require the use of a filter for each detector, each filter rejecting any radiation which is out of the wavelength band for the given detector. Accordingly, a substantial amount of energy can be lost, because radiation out of the band for the filter is discarded and not recovered, even though that radiation may be in the bandpass range for another adjacent filter which corresponds to a detector for a different wavelength range. Consequently, some such systems can suffer from a low fill factor (fraction of area of the detector which is photo-electrically active) and low efficiency, due to the loss of potentially detectable radiation within the broader wavelength range of interest.
Accordingly, improved methods and systems are desired which allow for the detection of multiple wavelength ranges or “colors” of radiation from a scene, to allow for real-time viewing of a representation of the scene.